Zooming from 18mm to 55m causes lens to darken (pictures taken are the same). This happens in both M and AF.

SOURCE: Lens mount plastic is broken on Lens

Unfortunately your lens is one of the kit versions which were built as cheaply as possible with a plastic lens mount. They were never intended to be repaired, but to be fair they usually seem to last quite well on the 18-55mm versions but the 18-105mm is heavier and longer and I've found that the plastic mount is particularly intolerant to any handling errors on this lens.

A professional repair will normally cost more than the lens is worth, but you can sometimes buy new replacement mounts.

Use your favourite search engine to look for part number 1C999-488. Unfortunately they're rarely in stock whenever I've needed one. You may also want to search for a scrap lens with a good mount which you can replace yours with, but scrap lenses of the correct type usually have faulty or badly worn plastic mounts as well.

Carefully assess your lens before making the repair: unless it's in perfect working order in all respects except for the broken mount then you'll be better off buying a new or used complete replacement lens.

SOURCE: Lens will only zoom in the range 24 - 40mm

Your motor may have gone bad. Try sending it to PhotoTech Repair Service in NYC. They are Nikon authorized, and can do the repair under warranty if you still have it. Also if you join they're facebook page, they will give you a 10% discount.
Here is the facebook page link, it has everything you need to send it in.
http://www.facebook.com/pages/New-York-NY/PhotoTech-Repair-Service-Inc/102527215194

SOURCE: Could I get the 18-105mm G ED DX VR AF-S lens

The D5000 is not a professional camera, it's very much a low-end introductory consumer grade digital SLR aimed at first timers. It's a fine camera, but wouldn't last more than a few days at best in professional use.

The camera usually comes bundled with a "kit" lens, such as the 18-55mm. Like the camera, it's a cheap and cheerful model which is really intended just to get you started. It costs very little to make, and like the camera body it's not designed with longevity, servicing or repairs in mind.

But to answer your question, yes, you can buy the D5000 as a body only option without the kit lens and then add on whichever lens you prefer at extra cost. The 18-105mm has a longer zoom range so has more flexibility than the 18-55mm, but once again it's very much a consumer grade lens. You don't get anything for nothing, so in return for the additional zoom range in a lens which is of a similar grade to the kit lens you get poorer image quality and really noticeable barrel distortion at the wide angle (18mm) setting and pronounced pin cushion distortion at anywhere above around 24mm (click here). You can't avoid these distortions entirely as all zooms are a compromise, but the kit lens suffers far less than the 18-105mm and also has a better all-round image quality (click here). Again, not to professional standards, but more than adequate for almost all amateur users. To be fair, although the 18-105mm appears to be based on an older design and is totally unacceptable for any professional use, it also is perfectly adequate for most amateurs and is relatively good value at the comparatively low price it can be purchased for, but the Nikkor AF-S 55-200mm f/4-5.6 G ED DX is by far a better lens altogether and just happens to have very low image distortion across the zoom range. If you can get one, it's the ideal companion to the kit lens. Best of all, it's actually cheaper than the one you propose getting (click here and here for comparison)and offers a much more useful telephoto range.

Please feel free to ask any questions if I've failed to answer your question in any way, otherwise, please take a moment to rate my answer.

Testimonial: "thanks .."

SOURCE: My AF-S Nikkor 70-300

Dust or dirt on the zoom track or damaged motor use in manual focus and get it fixed asap,you may damage your body if you AF motor has to strain to focus so maual focus it untill its fixed .

SOURCE: Nikon AF-s 28-70 1:2,8 zooming problem

There's really not very much an end user can do to adjust these lenses. They are precision mechanisms with small, moving parts and optics that all must remain free of dirt, dust and lubricants. Attempting to disassemble the lens could render it inoperable instantly - or down the road due to contaminants from handling.

Bring the camera and lens to a reputable camera shop if one is close by - or contact Nikon Factory Service to arrange for professional evaluation. Most Nikon lenses are protected from failure due to workmanship, etc. for 5 years with a copy of your dated sales slip (or other proof of purchase). If not covered by warranty due to age or handling, you will receive a price for repair and request for authorization for work to be completed. There is no charge for a repair estimate. You can choose not to make the repair and only be out the cost of round-trip shipping.

Good luck!